Ethylenimine, also referred to as aziridine, is an important fine chemical product, and has a wide application in the fields of medicine, pesticide, high-energy fuel, bonding agent, and the like.
Initially, ethylenimine is prepared industrially by liquid-phase method using monoethanolamine as a raw material and concentrated sulfuric acid as a dehydrating agent. During this reaction, it is necessary to add a concentrated alkaline, so that the cost is high and the useless by-products of inorganic salts are generated, which causes severe environment pollution.
A catalyst consisting of tungsten oxide and silicon oxide and the application thereof for the synthesis of ethylenimine are disclosed in U.S. Pat. No. 4,301,036.
A method of synthesizing ethylenimine by using a niobium/tantalum oxide and an alkaline earth metal oxide and (or) an iron/chromium oxide as catalysts is disclosed in U.S. Pat. No. 4,488,591. A method of preparing ethylenimine by using the complex oxide of niobium or tantalum oxide and an alkaline earth metal oxide (BaO) as a catalyst is disclosed in U.S. Pat. No. 433717. The method comprises the following steps: preparing the solution to be loaded from niobium halide or tantalum halide; loading the solution on the surface of the carrier (0.1 m2/g−1.0 m2/g), then roasting the loaded carrier at a temperature in a range of 450° C. to 550° C. for 2-4 hours to give the catalyst. The reaction temperature of preparing ethylenimine is 400° C., and the ethanolamine conversion rate is 13.87 (mol) % with a selectivity for ethylenimine of 82.09 (mol) %. In addition, a small amount of acetaldehyde, ethylene amine, pyrazine and alkylpyrazine are also generated.
A catalyst for synthesizing ethylenimine compounds is disclosed in CN2007100109625. The catalyst is XaYbOc(HZSM-5)d, wherein X is an alkaline metal, Y is phosphorus element or boron element, O is an oxygen element; a, b, c, and d are the molar ratios of the elements X, Y, O, and ZSM-5, respectively; and when d=1, a=0.01-0.1, b=0.001-0.1, and c is dependent on a and b. The reaction temperature of preparing ethylenimine is 420° C., and the ethanolamine conversion rate is 84 (mol) % with a selectivity for ethylenimine of 84 (mol) %.
The specific surface area and the acid-base intensity of a catalyst for ethylenimine are closely related to the activity and selectivity of the catalyst. Generally, the loaded catalyst would have a relatively high catalytic activity only when the specific surface area is relatively high. However, a roast step at a high temperature (≧600° C.) is needed to prepare such a catalyst. The local high temperature resulting from a strong exothermic reaction may significantly decrease the specific surface area of the roasted catalyst, resulting in the decrease in activity and selectivity of the catalyst. In view of the economic factors in the industrial processes, it is generally more important to seek for a catalyst with higher selectivity and longer service life than to improve the conversion rate of the materials, so as to ensure the conversion of the materials into the desired target products as more as possible and reduce the generation of the unwanted by-products.
In the prior art, the intramolecular dehydration reaction of amino alcohols is carried out at a relative low temperature (about 370° C.), and under such a low temperature, the activity and selectivity of the catalyst are relatively low. The reaction is generally carried out quickly at a high reaction temperature, generally above 400° C. However, the deamination and intramolecular condensation reactions of amino alcohols may occur easily at such a high temperature, and thus the by-products are increased, so that the selectivity for ethylenimine is decreased with the weight selectivity of the catalyst being lower than 65 wt %, and the catalyst has a short service life.